Corona generator

ABSTRACT

A tungsten wire corona generator having a stable coating thereon for providing the generator with an extremely long life and a stable output.

United States Patent Bonaventura et al.

Oct. 9, 1973 CORONA GENERATOR Inventors: Joseph J. Bonaventura,Rochester; Raymond J. Harshharger, Ontario,

both of NY.

Assignee: Xerox Corporation, Stamford,

Conn.

Filed: Dec. 23, 1971 Appl. No.: 211,542

US. Cl 317/262 A, ll7/231, 250/495 ZC,

Int. Cl. H0lt 19/00 Field of Search 317/2 R, 3, 4, 262 R, 317/262 A;313/354, 355; 250/495 ZC, 49.5 GC, 49.5 TC; 117/231 [56] ReferencesCited UNITED STATES PATENTS 2,922,883 l/l960 Giaimo, Jr. 317/262 A X3,537,914 ll/1970 Cieplinski et a1. 117/231 X Primary Examiner-J. D.Miller Assistant Examiner-Harry E. Moose, Jr. Attorney-James J. Ralabateet al.

[57] ABSTRACT A tungsten wire corona generator having a stable coatingthereon for providing the generator with an extremely long life and astable output.

12 Claims, 1 Drawing Figure CORONA GENERATOR This invention relates toan inexpensive corona generator capable of producing a stable anduniform output and having the relatively long operating life.

More specifically, this invention relates to an electrical coronagenerator capable of producing a highly efficient discharge thus is wellsuited to effectively charge a receiving surface such as a xerographicplate or the like. This novel generator includes a corona emitting wireconstructed of a tungsten material and having a surface coating thereonserving to extend the normal life of the generator and to smooth thecorona deposited upon the plate receiving surface.

Many methods and devices have been disclosed in the prior art forproducing a uniform electrostatic charge upon a photoconductive plate.One such charging device is disclosed by Vyverberg in U.S. Pat.2,836,725, issued May 27, 1958, wherein an electrode in the form of awire surrounded by an electrically grounded conductive shield is placedadjacent to a grounded receiving surface and a high voltage sourceconnected to the wire wherein a corona discharge is produced. A coronadischarge is produced when the generator is placed in close proximity tothe plate and applied to the generator of sufficient magnitude to causea breakdown of the ions in the air within the separating gap. As aresult, charged ions are formed around the corona generator flow to thegrounded plate surface and are deposited thereon to raise the platepotential to a relatively high level.

In the art of xerography it has been found that consistent reproductivequality can only be maintained when a uniform and constant chargepotential is applied to the photoconductive plate. In many automaticmachines of this type, a single wire generator, generally referred to asa corotron is employed. Generally, the efficiency of the corotron isdependent on many factors including the gap distance between the wireand the plate surface, the nature of the generating wire material, thediameter of the wire and other physical features thereof andthe amountof energy supplied to the corona emitter. Heretofore, most corotronswere constructed of platinum or some other precious metal because thesematerials displayed the ability to deliver uniform constant chargingover a long period of time. As a consequence, these prior art deviceswere relatively expensive to construct and replacement costs were alsohigh.

It is therefore an object of the present invention to provide a coronagenerating device constructed of a readily available inexpensivematerial.

A further object of this invention is to provide a relativelyinexpensive corona generator exhibiting a highly stable output andhaving a relatively long operating life.

A still further object of this inventin is to improve the coronagenerating art.

These and other objects of the present invention are attained by meansof apparatus for charging the surface of a photoconductive plate whichincludes a corona generating wire, a source of electrical energy beingoperatively connected to the generating wire to cause the wire to emit acorona discharge, the wire generator being constructed of a tungstenbase material having a high temperature oxide coating thereon which isextremely stable and non-spalling within the operating range of thegenerator.

For a better understanding of the present invention as well as otherobjects and features thereof, reference is had to the following detaileddescription of the invention to be read in connection with theaccompanying drawing wherein the drawing illustrates a perspective viewof a corona generating apparatus having a corona emitting wirefabricated in accordance with the teachings of the present invention.

Referring in particular to the attached FIGURE, there is illustrated acorona generating device containing a generator wire 25 of the presentinvention for depositing an electrostatic charge on the surface of amoving photoconductive recording material. Illustrated in the FIGURE isa xerographic plate 10 made up of a photoconductive insulating materialI] placed upon a grounded conductive substrate 12 with the plate beingarranged to move along a predetermined path of travel in the directionindicated. The corona generating unit, generally referred to as 15, ispositioned above the plate surface and is arranged to deposit anelectrical charge thereon as the plate surface moves in the directionindicated. The corona unit includes a shield member 17 whichsubstantially encloses one or more corona generator wires 25 19. Theshield is preferably made up of an electrically conductive material thatis placed at a ground potential. A slit 20 or opening is formed in thebottom of the shield opposite the moving photoconductive surface andprovides a path by which a flow of ions discharged by the generator aredirected towards and deposited upon the moving plate surface. Forfurther details regarding the structure of this type of coronagenerating unit, reference is had to the disclosure contained within theaforetomentioned Vyverberg patent.

The corona generating wire is connected by suitable means such aselectrical connector 21 to a high potential source 23. The corona wireutilized in the present embodiment is connected directly to the positiveterminal of the power source whereby a positive ion discharge is placedon the plate surface. However, it should be clear that an oppositearrangement can be employed to obtain a negative discharge. Basically,the generator is constructed of a fine wire between 0.003 and 0.006inches in diameter made of a high quality tungsten being 99.95 percentpure. The wire preferably has a strength of about 280,000 psi. minimumand be capable of being elongated 1.4 percent over a 10 inch length whentested by ASTM method F219 (1) utilizing a cross head speed ofapproximately 1 ii inches per minute.

Normally, a pure tungsten wire generator of this nature will initiallyexhibit extremely good corona generating properties. However, after avery short operating period, the uniformity of output, i.e. thedistribution and amount of corona generated by the wire, becomes erraticand the generator exhibits signs of early electrical breakdown. Thereason for this short operating life is thought to be caused by looserandomly dispersed tungsten oxides which form on the wire surface at therelatively low operating temperatures. These low temperature oxides arefound to be metastable and correspondingly, cause an unstable dischargeto be produced by the generator. Furthermore, the mechanical propertiesof these low temperature oxides are such that the oxides invariablyspall from the wire in an uncontrolled manner further aggravating theproblems involved.

In the present invention, all the shortcomings previously associatedwith a tungsten wire corona generator have been overcome to provide agenerator which is inexpensive to produce and which has an extremelylong operating life and the capability of delivering an extremelyuniform stable output over its entire operating life. To produce thistruly novel result, the generator wire is specially treated to form anextremely smooth, uniform and tough coating thereon which will remainrelatively stable throughout the operating life of the generator.

To produce this generator, a substantially pure tungsten wire 18 havingthe beforementioned properties is chemically cleaned to remove foreignmatter and grease from the surface of the wire and the wire stressrelieved. Any suitable process commonly known and used in the art can beherein employed. The wire is then placed within an electrical oven andthe oven heated to a temperature in excess of 600C in air. A wire havinga diameter of approximately 0.0035 inches, when heat treated in thismanner for approximately minutes, will be provided with an oxide coating19 which is between 1.5 and 3.5 microns thick. An analysis of thiscoating shows that it is made up of high temperature oxides including W0W0 and W 0 Generating wires produced in this manner have been found tohave an extremely stable coating which serves to prevent physicalchanges in the wire at the operating temperature and provides for anextremely stable output and long operating life. The high temperatureoxide coating thus formed is relatively tough and is capable of readilywithstanding light scraping with a metallic instrument. Spalling of thecoating occurs only when the generator wire is permanently deformed orkinked. In fact, the wire can be bent about a A inch diameter rodwithout disturbing the coating in any manner.

In order to ascertain the capabilities of the generator of the presentinvention, a test fixture was constructed by which the output of thegenerator could be evaluated and compared to similar generatorsconstructed of other materials. The fixture basically consisted of arotatably mounted grounded xerographic plate in drum configurationhaving a corona wire support mounted adjacent thereto for supporting awire generator transverse to the drum surface. The wire generator wasarranged to be electrically coupled to a high power source of electricalenergy with the wire being supported above the drum surface at adistance whereby the corotron drew approximately 325 micro amps ofcurrent while charging the plate surface to a potential of approximately800 volts. A thin brass strip was helically wound about the drum so thatthe strip continually monitored the bare plate current flow between thegenerator wire and the plate as the drum rotated under the generator.The brass strip was electrically brought out to a recording device bymeans of a slip ring and the recorder arranged to record data atapproximately mm/sec. Under these operating conditions, a bare platecurrent of approximately 50 micro amps was initially recorded and theamplitude of the current deviation from this initial current reading wasgraphically registered.

A heat treated tungsten wire was placed within the test apparatus andcontinually operated for a period of approximately 250 hours. Periodictraces of the bare plate current reveal that the output of the heattreated generator remain relatively stable and unchanged over the testperiod. Subsequent to this test, a second corona generatorpf untreatedtungsten wire similar in dimensions to that previously tested was placedwithin the fixture and an operating current placed thereon. Afterapproximately 25 minutes of operation, the untreated tungsten generatorbegan to produce a non-uniform and unstable output and thereafter thegenerator continued to electrically fail.

Next a comparison test of a heat treated tungsten generator and aplatinum generator having similar physical properties was made. Thewires were properties arranged in the fixture so as to produce the samebare plate current output at the drum surface. After approximately 256continuous hours of operation, no differences in the performance of theplatinum wire generator and that of the treated tungsten wire generatorcould be discerned.

While this invention has been described with reference to the structureherein disclosed, it is not confined to the details as set forth andthis application is intended to cover any modifications or changes thatmay come within the scope of the following claims.

What is claimed is:

l. A device for emitting a discharge of corona including a tungstenelectrode being arranged to emit a corona discharge,

a coating overlying at least the discharge region of said electrodebeing formed of a high temperature tungsten oxide that is stable duringperiods when said electrode is emitting corona whereby a uniformdischarge of corona is produced over an extended period of time.

2. The apparatus of claim 1 wherein the tungsten electrode is formed ofa material that is at least 99 percent pure.

3. The apparatus of claim 2 wherein the oxide formed on said electrodeincludes the oxides WO and W 0 4. The apparatus of claim 3 wherein theoxide coating is of a thickness of about or greater than one-half amicron.

5. The method of producing a corona emitting electrode which is capableof producing a stable output over an extended period of time includingproviding an electrode of relatively pure tungsten,

and

oxidizing said electrode by heating the electrode in air to atemperature at or about 600C for a period of time Sufl'Tcient ttEFattfiionina generating surface with a tungsten oxide layer.

6. The method of claim 5 wherein the coating formed on said electrodeincludes the oxides W0, and W 0 7. The method of Claim 5 wherein saidelectrode is precleaned prior to oxidation to remove impurities from thesurface of said electrode.

8. The method of claim 5 wherein said electrode is heated at or above600C for at least 10 minutes.

9. The method of claim 5 wherein said oxide coating is formed to athickness which is at least one-half of a micron.

10. The method of claim 5 wherein said electrode is formed of a tungstenmaterial which is at least 99 percent pure.

11. The method of producing a corona emitting electrode that is capableof producing a stable output over an extended period of time includingproducing an elongated tungsten wire formed of a relatively purematerial,

cleaning the surface of said tungsten wire to remove impuritiestherefrom,

oxidizing said tungsten wire by heating said wire in air to atemperature at or above 600C fora period of time sufficient to form anoxide coating thereon 5 w18O49- which is between one-half and three andone-half

2. The apparatus of claim 1 wherein the tungsten electrode is formed ofa material that is at least 99 percent pure.
 3. The apparatus of claim 2wherein the oxide formed on said electrode includes the oxides WO2 andW18O49.
 4. The apparatus of claim 3 wherein the oxide coating is of athickness of about or greater than one-half a micron.
 5. The method ofproducing a corona emitting electrode which is capable of producing astable output over an extended period of time including providing anelectrode of relatively pure tungsten, and oxidizing said electrode byheating the electrode in air to a temperature at or about 600*C for aperiod of time sufficient to coat the corona generating surface with atungsten oxide layer.
 6. The method of claim 5 wherein the coatingformed on said electrode includes the oxides WO2 and W18O49.
 7. Themethod of Claim 5 wherein said electrode is precleaned prior tooxidation to remove impurities from the surface of said electrode. 8.The method of claim 5 wherein said electrode is heated at or above 600*Cfor at least 10 minutes.
 9. The method of claim 5 wherein said oxidecoating is formed to a thickness which is at least one-half of a micron.10. The method of claim 5 wherein said electrode is formed of a tungstenmaterial which is at least 99 percent pure.
 11. The method of producinga corona emitting electrode that is capable of producing a stable outputover an extended period of time including producing an elongatedtungsten wire formed of a relatively pure material, cleaning the surfaceof said tungsten wire to remove impurities therefrom, oxidizing saidtungsten wire by heating said wire in air to a temperature at or above600*C for a period of time sufficient to form an oxide coating thereonwhich is between one-half and three and one-half microns thick.
 12. Themethod of claim 11 wherein the oxide coating formed on said wireincludes the oxides WO2 and W18O49.